It's mine: Sultan Jamalul Kiram III explains his claim to reporters at his home in Manila. Photo: AP

Over several days more intruders are believed to have arrived by boat and spread out into other villages along the Sabah coast, triggering Malaysia's most serious security crisis in decades, which has left 25 people dead in two shootouts.

The man reportedly behind the surreal events is ailing 74-year-old Jamalul Kiram III, the Sultan of Sulu, who claims to reign over an archipelago at the remote southernmost tip of the Philippines, although the territory is not recognised by any state. Nor is Mr Kiram's family the only one that has claimed to be the rightful sultan.

Mr Kiram greets reporters sitting on a dirty plastic chair at his two-storey house in a rundown district of Manila, telling them his followers have gone to Sabah to reclaim their land.

''If they have to die, then they will die,'' he said, though insisting ''the door of the sultanate for negotiation is open''.

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The row over Sabah is old and convoluted. In 1658 the Sultan of Brunei ceded the area to the Sultan of Sulu for helping to quash a rebellion. The Sultan of Sulu later leased Sabah to the British North Borneo Company.

At the end of the colonial era the British folded the territory into what was then called the Federation of Malaya in 1963, instead of handing Sabah back to the Sulu sultanate, by then part of the Philippines.

Five years later then Philippine dictator Ferdinand Marcos hatched a plan to invade and annex Sabah but was forced to abort it after Muslim Philippine troops rebelled.

They were massacred, triggering an uprising in the Muslim-dominated southern Philippines.

For decades Malaysia has made modest ''cession'' payments of about $US1500 a year to the heirs of the sultanate, in apparent recognition of the territory's contested absorption.

Mr Kiram has repeatedly called on successive Philippine governments to support his historical claim to Sabah, saying he has documents from the 1800s to prove it.

On November 1, Mr Kiram issued a decree mandating his followers to travel to and settle peacefully in Sabah.

It seems he was unhappy at being excluded from a peace agreement between the Philippine government and the Moro Islamic Liberation Front (MILF), the largest Muslim rebel group in the southern Philippines, brokered by Malaysia and signed the previous month.

Few took notice of the decree until the men arrived on Sabah.

The subsequent standoff has the potential to strain ties between Malaysia and the Philippines, although Philippine President Benigno Aquino has repeatedly called on Mr Kiram's followers to withdraw and sent navy patrols to the area to try to prevent reinforcements joining them.

Sabah shares a long sea border with the Philippines that is difficult to patrol.

Mr Aquino has in the past described the Sabah claim as ''dormant'' but in the past few days Philippine officials have suggested the country could re-examine it.

Officials fear that if the sultan's group is martyred in an armed crackdown, his followers could emerge as a third armed group in the southern Philippines and undermine the MILF agreement.

More than 800,000 Philippine workers, many of them undocumented, live in Sabah, which has oil and gas fields. Chinese companies are investing in hydro-electricity and coalmines there.

Mr Kiram told reporters he was worried the violence might spread because many Filipinos, especially followers of his sultanate in the southern Philippines, are upset at the killing of 12 of their compatriots in a firefight last Friday.

His daughter, Jacel, who describes herself as a princess, has called on Filipinos to stay calm but stressed the sultanate would never back down from its struggle to reclaim Sabah.

''This concerns honour above life,'' she said.

''We will not retreat just like that, because we're fighting for something and our struggle is our right and truth.''